Programmable Device Enables Optical Control at Unprecedented Speeds

An international team led by MIT researchers has developed a spatial light modulator (SLM) that promises greater control of light at orders of magnitude more quickly than commercial devices. The team also developed a fabrication process to ensure consistent device quality when manufactured at scale.

The device could be used to create superfast lidar sensors for self-driving vehicles that could image a scene about a million times faster than existing mechanical systems. It could also be used to accelerate brain scanners that use light to see through tissue. By being able to image tissue faster, the scanners could generate higher-resolution images that aren’t affected by noise from dynamic fluctuations in living tissue, like flowing blood.

Spatial light modulators manipulate light by controlling its emission properties. Similar to an overhead projector or computer screen, an SLM transforms a passing beam of light, focusing it in one direction or refracting it to many locations for image formation. Inside the SLM, a two-dimensional array of optical modulators controls the light. However, since wavelengths are only a few hundred nanometers, so to precisely control light at high speeds, the device needs an extremely dense array of nanoscale controllers.

In the recent work, the researchers used an array of photonic crystal microcavities to achieve this goal. These photonic crystal resonators allow light to be controllably stored, manipulated, and emitted at the wavelength scale.

Scientists have developed a programmable, wireless spatial light modulator that can manipulate light at the wavelength scale with orders of magnitude faster response than existing devices. Courtesy of Sampson Wilcox/MIT.

In the work, by varying the reflectivity of a cavity, the researchers controlled how light escaped. Simultaneously controlling the array modulated an entire light field — so the researchers could quickly and precisely steer a beam of light.

Christopher Panuski, lead author of the paper and a recent Ph.D. graduate, said, “One novel aspect of our device is its engineered radiation pattern. We want the reflected light from each cavity to be a focused beam because that improves the beam-steering performance of the final device. Our process essentially makes an ideal optical antenna.”

The researchers developed an algorithm to design photonic crystal devices that form light into a narrow beam as it escapes each cavity, Panuski said.

The team used a micro-LED display to control its SLM. The LED pixels lined up with the photonic crystals on the silicon chip, so that turning on one LED tuned a single microcavity. When a laser hit that activated microcavity, the cavity responded differently to the laser based on the light from the LED.

The use of LEDs to control the device ensured that the array is not only programmable and reconfigurable, but also completely wireless, Panuski said. “It is an all-optical control process. Without metal wires, we can place devices closer together without worrying about absorption losses.”

To fabricate the device, the researchers aimed to use the same techniques that create integrated circuits for computers, thereby allowing them to mass-produce the device. They partnered with the Air Force Research Laboratory to develop a highly precise mass-manufacturing process that stamps billions of cavities onto a 12-in. silicon wafer. Then they incorporated a post-processing step to ensure the microcavities all operate at the same wavelength.

“Getting a device architecture that would actually be manufacturable was one of the huge challenges at the outset," said Dirk Englund, senior author and associate professor of electrical engineering and computer science at MIT. According to Englund, the fabrication approach benefitted from a technique for machine-vision based holographic trimming developed by Panuski. For this “trimming” process, the researchers shined a laser onto the microcavities. The laser heated the silicon to more than 1000 ºC, creating silicon dioxide. The researchers created a system that blasted all the cavities with the same laser at once, adding a layer of glass that perfectly aligned the resonances — that is, the natural frequencies at which the cavities vibrate.

The device demonstrated near-perfect control — in both space and time — of an optical field with a joint spatiotemporal bandwidth 10× greater than that of existing SLMs. Being able to precisely control a huge bandwidth of light could enable devices that can carry massive amounts of information extremely quickly, such as high-performance communications systems.

Now, the researchers are working to make larger devices for quantum control or ultrafast sensing and imaging.

The paper was a collaboration between researchers at MIT, Flexcompute Inc., the University of Strathclyde, the State University of New York Polytechnic Institute, Applied Nanotools Inc., Rochester Institute of Technology, and the U.S. Air Force Research Laboratory.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-022-01086-9).